1. Field of the Invention
This invention relates to centrifugal airfoil fan wheels, more particularly, this invention relates to a method for modifying a centrifugal airfoil fan wheel for improved performance and to prevent operational low cycle fatigue and the airfoil fan produced thereby.
2. Description of the Prior Art
Centrifugal airfoil fans are well known. Centrifugal airfoil fans are typically constructed of various materials and are designed to have an aerodynamic shape having varying wheel diameter and width depending upon the operational characteristics desired. Optimal design characteristics for centrifugal fans include reduced vibration, increased efficiency and reduced centrifugal force stresses. These design characteristics facilitate the practical use of smaller mounting shafts and bearings and smaller rated drive motors.
In any centrifugal fan design, the centrifugal fan, during operation, will experience steady state stresses which are relatively high when compared to the yield strength of the material utilized to make the fans. The steady state stresses are attributable to the combined twisting, bending and in-plane and torsional loads imposed upon the centrifugal fan wheels during operation. As a result of these operational stresses, many centrifugal fans will at some point in time experience low cycle fatigue which initially manifests itself in localized cracking, then crack propagation and eventual wheel failure. The low cycle fatigue condition is exacerbated by the frequent starting and stopping of the wheel, speed changes, and erosion and corrosion experienced by centrifugal fans. The operating life of the fan will be short and the fan may fail catastrophically when the stresses due to operation are very high relative to the yield strength of the material from which the fan is constructed. Because low cycle fatigue in centrifugal airfoil fans is cumulative and irreversible, it has been an accepted practice in the industry to replace a failing fan wheel thereby causing operational delays and additional expenses. Accordingly, applicant has invented a method for retrofitting a failing centrifugal airfoil fan wheel thereby allowing for the safe continued use of the fan wheel at a minimum loss of time and expense. A centrifugal fan wheel modified in accordance with the present invention is capable of operating at optimum efficiency and performance for an extended period of time.
It is an object of the present invention to provide an inexpensive method for retrofitting a failing centrifugal airfoil fan wheel.
It is yet another object of the present invention to provide a modified centrifugal airfoil fan wheel with lower operating stress levels than that of an unmodified airfoil fan wheel.
Another object of the present invention is to provide a modified centrifugal airfoil fan wheel of a sturdy construction having a reduced or substantially the same weight and inertia with lower operating stress levels than that of an unmodified airfoil fan wheel.
Another object of the present invention is to provide a modified centrifugal airfoil fan wheel capable of operating for an extended period of time.